This was in the Linux kernel, which is compiled with special kernel flags which make dereferencing null pointers legal. In the context of that code, dereferencing a pointer and later checking it for null was absoutely meaningful. Optimizing the later null check was a compiler bug that was acknowledged and fixed. The compiler here didn't respect the semantics it had promised to kernel code. This was entirely uncontroversial; it was not a case of "unwanted optimization based on undefined behavior", it was a case of "compiler bug breaking well-defined code". Again, all this in a kernel context.

In user code GCC will still happily remove the null check because in user code this is an actual bug in the user's code.